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Flu (Influenza) Antigenic Shift

courtesy of the National Institute of Allergy and Infectious Diseases

The genetic change that enables a flu strain to jump from one animal species to another, including humans, is called antigenic shift. Antigenic shift can happen in three ways:

Antigenic Shift 1 - A duck or other aquatic bird passes a bird strain of influenza A to an intermediate host such as a chicken or pig. A person passes a human strain of influenza A to the same chicken or pig. When the viruses infect the same cell, the genes from the bird strain mix with genes from the human strain to yield a new strain. The new strain can spread from the intermediate host to humans.

Antigenic Shift 2- Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to humans.


Antigenic Shift 3- Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to an intermediate animal host and then to humans. The new strain may further evolve to spread from person to person. If so, a flu pandemic could arise.